1. Field of The Invention
The present invention relates to a process for the preparation of solutions of hardenable urea-formaldehyde resins, especially of resins for impregnating paper supports used for coating wood-based panels.
2. Description of The Prior Art
There is an extensive literature that deals with processes for the preparation of urea-formaldehyde precondensation resins and their properties. Special reference is made in this connection to the summarizing monograph of J. Scheiber, "Chemie und Technologie der kunstlichen Harze (Chemistry and Technology of the Synthetic Resins)", 1943 edition, page 333 ff. and to the corresponding section in Houben-Weyl, volume 14/2, page 319 ff., 1963 edition.
However, the urea-formaldehyde resins known conventionally have a series of disadvantages. These disadvantages are especially evident from the fact that urea resins harden relatively slowly in the pH region above about 4, while at a pH region below 4 then tend, to harden more rapidly and, in fact, relatively precipitously and therefore, uncontrollably.
If compounds, such as, latent hardeners are used which lead to strongly acidic reactions, hardened products are obtained which, as a result of excessively rapid hardening, are very brittle and, in cases where they are used for coating the surface of wood-based panels, produce surfaces which tend to crack. If, however, hardeners are used which lead to weakly acidic reactions, such as, for example, most of the amine salts of organic acids, one must be prepared to put up with a relatively long hardening times or high hardening temperatures. Even then, products are obtained in many cases which contain portions of resin which have not been hardened, in addition to hardened duroplastic polycondensation portions.
This hardening behavior is associated with a series of disadvantages from an applications point of view. Thus, resins hardened with strongly acidically acting latent hardeners tend to be brittle and do not produce satisfactory surfaces when wood-based panels are coated with these resins at processing temperatures above 120.degree. C. because of the rapid hardening. Moreover, resins hardened with amine salts of carboxylic acids have little resistance to the detrimental effects of water and temperature, since the proportion of resin, which has not completely hardened is relatively large.
Attempts to improve the duroplastic properties of urea-formaldehyde resins by increasing the hardening temperature have failed because of the decomposition of urea-formaldehyde resins, which clearly becomes evident at temperatures from 120.degree. C. upwards. The above-described hardening characteristics of urea-formaldehyde resins interfere particularly with the use of these resins for coating surfaces in so-called short-contact presses, in which the urea-formaldehyde resin can be exposed for short times to temperatures up to 150.degree. C. The hardening times conventionally used are too short for converting the resin completely to the duroplastic state. On the other hand, the press temperatures are already so high that the thermal instability of the urea-formaldehyde resin becomes a problem.
In this regard, German Patent Application No. P 24 48 472.8 (Belgian Patent No. 834,032) is noted which involves a process for the preparation of urea-formaldehyde solutions wherein an aqueous solution prepared according to the following procedure is used:
(a) urea and formaldehyde, in a mole ratio of 1 : 1.5 to 2.5, are reacted in the presence of 0.2 to 1.0 mmoles of an aminosulfonic acid and 20 to 100 mmoles of ammonia (in each case based on 1 mole of urea) at temperatures of 70.degree. to 95.degree. C. for 10 to 30 minutes, until the 50% solution has a viscosity of 55 to 65 cP at 20.degree. C.;
(b) 0.8 to 10 mmoles of an aminosulfonic acid are then added, a pH of 4.0 to 4.5 is maintained with ammonia during the reaction time of 10 to 25 minutes at 70.degree. to 95.degree. C., until the 50% solution has a viscosity at 20.degree. C. of 80 to 110 cP; and finally,
(c) 40 to 200 mmoles of ammonia, as well as 0.1 to 0.3 moles of urea are added to this reaction product and the reaction mixture is converted in 15 to 45 minutes at a temperature of 70.degree. to 95.degree. C. until the 50% solution has a viscosity of 85 to 125 cP at 20.degree. C.
While the purpose of the above process is to overcome the above noted difficulties of the prior art, there are, however, a number of problems with this procedure especially in the case of large batches. Thus, the reaction of Step (a) is rather difficult to control because of the exothermic nature of the process, especially when, for example, because of the equipment an efficient heat dispersion cannot be achieved without difficulty. It was furthermore discovered that, for Step (b), maintaining the pH at 4.0 to 4.5 creates difficulties, since the pH has a tendency to decrease, so that a constant, controlled addition of ammonia is required. This, however, impairs the simplicity of the aforementioned process.